1. Field of the Invention
The present invention relates to a power control unit and power control method, more particularly to a power supply in which a secondary battery and a device are connected in parallel to output.
2. Description of the Related Art
Universal serial bus (USB) used as the general purpose interface for personal computer and IEEE1394 include a power supply line as well as an interface means. However, there is a limit in power supplied from a power supply line. For example, power supplied by the USB interface has a limit of current of 5V/500 mA. If a power over this limit value is requested by a device side, a protective circuit is actuated so as to shut down supply of power to the device. However, some of devices connected to the interface bus, such as a hard disk drive and DVD device, require a large consumption current transiently in a short period, for example, when its motor is started.
There exists a power supply in which a secondary battery and a device are connected in parallel to its output in order to operate the device stably. In such a power supply, when power which can be supplied from the interface bus has a margin, the secondary battery is charged by this margin of power. Then, when the power used by the device exceeds the power supply capacity of the interface bus, the shortage power is supplied to the device from the secondary battery.
As related arts, Japanese Unexamined Patent Publication Nos. 2004-288537, 2005-12889, 2005-275611, H10 (1998)-323026, 2000-29544, and H8 (1996)-182219 have been disclosed.